Question

Let E be a given directed edge set. Suppose it is known that the edges in E can form a directed tree T with all the nodes (except the root node) has only 1 in-degree. The problem is how to efficiently traverse the edge set E, in order to find all the paths in T?

For example, Given a directed edge set E={(1,2),(1,5),(5,6),(1,4),(2,3)}. We know that such a set E can generate a directed tree T with only 1 in-degree (except the root node). Is there any fast method to traverse the edge set E, in order to find all the paths as follows:

Path1 = {(1,2),(2,3)}
Path2 = {(1,4)}
Path3 = {(1,5),(5,6)}

By the way, suppose the number of edges in E is |E|, is there complexity bound to find all the paths?

Answer 1

I have not worked on this kind of problems earlier. So just tried out a simple solution. Check this out.

public class PathFinder
{
    private static Dictionary<string, Path> pathsDictionary = new Dictionary<string, Path>();
    private static List<Path> newPaths = new List<Path>();
    public static Dictionary<string, Path> GetBestPaths(List<Edge> edgesInTree)
    {
        foreach (var e in edgesInTree)
        {
            SetNewPathsToAdd(e);
            UpdatePaths();
        }
        return pathsDictionary;
    }        
    private static void SetNewPathsToAdd(Edge currentEdge) 
    {
        newPaths.Clear();
        newPaths.Add(new Path(new List<Edge> { currentEdge })); 
        if (!pathsDictionary.ContainsKey(currentEdge.PathKey()))
        {
            var pathKeys = pathsDictionary.Keys.Where(c => c.Split(",".ToCharArray())[1] == currentEdge.StartPoint.ToString()).ToList();
            pathKeys.ForEach(key => { var newPath = new Path(pathsDictionary[key].ConnectedEdges); newPath.ConnectedEdges.Add(currentEdge); newPaths.Add(newPath); });             
            pathKeys =  pathsDictionary.Keys.Where(c => c.Split(",".ToCharArray())[0] == currentEdge.EndPoint.ToString()).ToList();
            pathKeys.ForEach(key => { var newPath = new Path(pathsDictionary[key].ConnectedEdges); newPath.ConnectedEdges.Insert(0, currentEdge); newPaths.Add(newPath); });
        }            
    }
    private static void UpdatePaths()
    {
        Path oldPath = null;
        foreach (Path newPath in newPaths)
        {
            if (!pathsDictionary.ContainsKey(newPath.PathKey()))
                pathsDictionary.Add(newPath.PathKey(), newPath);
            else
            {
                oldPath = pathsDictionary[newPath.PathKey()];
                if (newPath.PathWeights < oldPath.PathWeights)
                    pathsDictionary[newPath.PathKey()] = newPath;
            }
        }
    }        
}

public static class Extensions
{
    public static bool IsNullOrEmpty(this IEnumerable<object> collection) { return collection == null || collection.Count() > 0; }
    public static string PathKey(this ILine line) { return string.Format("{0},{1}", line.StartPoint, line.EndPoint); }
}
public interface ILine 
{
    int StartPoint { get; }
    int EndPoint { get; }
}
public class Edge :ILine 
{
    public int StartPoint { get; set; }
    public int EndPoint { get; set; }

    public Edge(int startPoint, int endPoint)
    {
        this.EndPoint = endPoint;
        this.StartPoint = startPoint;
    }
}
public class Path :ILine
{
    private List<Edge> connectedEdges = new List<Edge>();
    public Path(List<Edge> edges) { this.connectedEdges = edges; }
    public int StartPoint { get { return this.IsValid ? this.connectedEdges.First().StartPoint : 0; } }
    public int EndPoint { get { return this.IsValid ? this.connectedEdges.Last().EndPoint : 0; } }
    public bool IsValid { get { return this.EdgeCount > 0; } }
    public int EdgeCount { get { return this.connectedEdges.Count; } }
    // For now as no weights logics are defined
    public int PathWeights { get { return this.EdgeCount; } }
    public List<Edge> ConnectedEdges { get { return this.connectedEdges; } }
}